As portable electronic devices, hybrid vehicles, etc., have higher performance in recent years, secondary batteries (particularly lithium ion secondary batteries) used therein are increasingly required to have higher capacity. For current lithium ion secondary batteries, the development of higher-capacity positive electrodes is behind the development of higher-capacity negative electrodes. Even lithium nickel oxide-based materials, which are said to have relatively high capacity, have a capacity of about 190 to 220 mAh/g.
In contrast, sulfur, which has a theoretical capacity as high as about 1,670 mAh/g, is a promising candidate for a high-capacity electrode material. However, elemental sulfur has low electronic conductivity, and does not contain lithium; therefore, lithium or lithium-containing alloys must be used in negative electrodes. Thus, there are problematically fewer options for negative electrodes.
Comparatively, lithium sulfide, which contains lithium, allows for the use of graphite, silicon, and other alloys in negative electrodes, providing many more options for negative electrodes. Further, dangers such as short circuit due to the production of dendrites caused by the use of metallic lithium can be avoided. However, lithium sulfide also has the problem of low electronic conductivity, and it is known that charge and discharge hardly occur only by mixing lithium sulfide with a carbon powder, which is a conductive material (Non-Patent Document 1, listed later). For this reason, techniques of imparting electronic conductivity to lithium sulfide are essential to improve the performance of sulfur-based positive electrode materials.
In an attempt to increase the electronic conductivity of lithium sulfide, Non-Patent Documents 2 and 3, listed later, report a method of combining lithium sulfide with a copper powder, which is used as a conductive material. According to this method, however, such a copper powder and other transition metal powders, which are generally heavier than lithium sulfide, may cause a reduction in battery energy density per weight. Additionally, copper powder is more expensive per weight than carbon powder, resulting in increased costs of batteries. Therefore, carbon powder is more advantageous from the standpoint of industrial production; however, there have been no reports on the combination of lithium sulfide with carbon.
As for the combination of elemental sulfur with carbon, for example, Patent Document 1, listed later, discloses a method of subjecting sulfur and carbon to mechanical milling in an air atmosphere; however, there have been no reports on the production of lithium sulfide-carbon composites by such a dry process. Further, Patent Document 2, listed later, reports a method for depositing lithium polysulfide (e.g., Li2S12) on carbon by liquid-phase reaction; however, there have been no reports on a method of combining lithium sulfide (Li2S) with carbon.